I think it's usually the hole where they insert the servo pattern heads.
The servo pattern is the 'formatting' of the drive that help the drive know where the head is during seeks, so it can gauge acceleration and deceleration for getting to the next track, and for it to settle on the exact correct track once it gets near. There are several radial arms that slice the disc into sectors for radial positioning as well.
The drive heads aren't used for servo writing. Servo heads are generally external and inserted into the drive, allowing for higher priced, better precision parts to be used in servo writing, as the servo pattern is vital for drive operation.
I think some manufacturers are looking at using other methods of making servo patterns, but I don't think any have completely eliminated the servo writing step for 100% of it's drives.
At least that's my understanding.